This application is related to commonly owned U.S. Application entitled “A System and Method of Transfer Printing an Organic Semiconductor,” Ser. No. 10/314,632, filed herewith on the same day, cross-referenced and incorporated herein by reference. This application is also related to the commonly owned U.S. Application entitled “Synthesis and Application of Photosensitive Pentacene Precursor in Organic Thin Film Transistors,” Ser. No. 10/323,899, filed Dec. 20, 2002.
1. Field of the Invention
The present invention relates to a substrate having thereon a small molecule organic semiconductor layer. The present invention also relates to methods and systems for the production of such substrates having patterned small molecule organic semiconductor layers thereon. The patterned small molecule organic semiconductor layer is formed by heating a predeposited precursor of an organic small molecule residing on a substrate. More particularly, the present invention relates to a substrate having thereon a pentacene semiconductor layer.
2. Description of the Prior Art
Thin-film transistors and other electronic devices using organic semiconductors are emerging as alternatives to established methods using amorphous silicon (α-Si:H) as the semiconductor.
A variety of organic compounds have been proposed and tested as semiconducting materials for thin film transistor (TFT) devices. For example, among the p-channel (hole transport) materials that have been characterized are thiophene oligomers proposed as organic semiconductor material for TFT in Garnier, F., et al., “Structural basis for high carrier mobility in conjugated oligomers” Synth. Meth., Vol. 45, p. 163 (1991), and phthalocyanines described in Bao, Z., et al., “Organic Field-effect transistors with high mobility based on copper phthalocyanine” Appl. Phys. Lett., Vol. 69, p. 3066 (1996). Pentacene, which is a member of poly(acene) compounds has been proposed as an organic semiconductor material in Lin et al. IEEE 54th Annual Device Research Conference, 1996, pages 2136–2139, and Dimitrakopoulos et al., J. Appl. Phys. Vol. 80 (4), 1996, pages 2501–2507.
Some soluble organic compounds have also been characterized as organic semiconducting materials. For example poly(3-alkylthiophene) described in Bao, Z., et al., “Soluble and Processable regioregular poly(3-hexylthiophene) for thin film field-effect transistors application with high mobility” Appl. Phys. Lett., Vol. 69, page 4108 (1996).
An attractive material would have a high carrier mobility which is close to that of amorphous silicon (0.1–1 cm2.V−1.Sec−1), with a very high on/off ratio (>105). For an organic material to replace amorphous silicon, not only would it have the electrical properties cited above but also it should be processable from solution so that it could become commercially attractive.
Among the organic compounds which have been studied as semiconductors, only regioregular poly(3-hexylthiophene) is readily soluble in organic solvents and thin films of this compound have been processed from solution for construction of TFTs. The drawback of this compound is that it has relatively low (5×10−2 cm2.V−1.s.−1) carrier mobility and even much less satisfactory on/off ratio of less than 100. In addition, because thin films of this polymer are not stable in air and their field-effect characteristics deteriorate on exposure to air, its application as a semiconductor becomes less desirable.
The best performance as a semiconductor among organic materials to date has been achieved by thin films of pentacene deposited under high vacuum and temperature as reported by Dimitrakopoulos et al., in U.S. Pat. Nos. 5,946,511; 5,981,970 and 6,207,472 and other publications by Brown et al., J. Appl. Phys. 80(4), 1996, pages 2136–2139 and Dimitrakopoulos et al., J. Appl. Phys. 80 (4), pages 2501–2507.
Recently, thin-film transistors on plastic substrates using evaporated films of pentacene as the p-channel carrier with mobility of 1.7 cm2.V−1.s.−1 and an on/off ratio of 108 have been reported by Jakson et al., in Solid State Technology, Vol. 43 (3), 2000, pages 63–77.
Thin films of pentacene are very stable in air and even moderate temperatures and as far as performance is concerned, pentacene is the most attractive organic material to replace amorphous silicon.
The drawback up to now for pentacene is that it is insoluble in common organic solvents and it can only be deposited as a thin film by expensive high vacuum and temperature techniques.
There has been very little effort for the synthesis of soluble pentacene derivatives and the only example of soluble pentacene known to us is by Muellen, K. et al., “A soluble pentacene precursor: Synthesis, solid-state conversion into pentacene and application in a field-effect transistor,” Adv. Mat. 11(6), p. 480 (1999), in which a precursor of pentacene is synthesized by a tedious multi-step synthetic approach. The derivative, which is soluble in organic compounds and can be processed from solution, is converted back to pentacene by heating at moderate temperatures (140–200° C.).
The drawback in using this compound as a pentacene precursor is that due to multi-step synthesis (more than 9 steps), its preparation, especially in large a scale, is almost impractical. In addition, its conversion to pentacene occurs at a relatively high temperature, which prevents the use of most plastics as substrates.
Commonly owned and copending application entitled “Hetero Diels-Alder Adducts of Pentacene as Soluble Precursors of Pentacene,” Ser. No. 10/300,645, Filed on Nov. 20, 2002, contents of which are incorporated herein by reference, describes a specially prepared pentacene precursor that can be spun, dipped, or sprayed onto a substrate from which a small molecule organic semiconductor can result from simple thermal processing of the precursor. The precursor, after application to a substrate, is then allowed to dry. Upon heating the substrate (upon which the dried precursor film resides) on a hot plate at temperatures of 200° C. or less for several minutes or less the precursor has been shown to transform into a pure small molecule organic semiconductor, such as pentacene. Commonly owned application entitled “Thin Film Transistors Using Solution Processed Pentacene Precursor as Organic Semiconductor,” Ser. No. 10/300,630, filed on Nov. 20, 2002, and issued as U.S. Pat. No. 6,963,080, contents of which are incorporated herein by reference, describes the application of a solution processed polycyclic aromatic compound precursors as an organic semiconducting material in thin film transistors.
The identity of the film after thermal processing has been verified as the small molecule organic semiconductor by infrared reflectance-absorption spectroscopy. With a localized or focused heat or energy source that can move relative to the precursor deposited on an energy absorbing substrate, a patterned small molecule organic semiconductor layer can be obtained. An ideal heat or energy source to produce the heat required for patterning is a scannable focused laser with the light absorbed by an energy absorbing film, such as a thin film, in contact with the precursor or an energy absorbing substrate onto which the precursor has been deposited where the substrate is light absorbing.
Yet another configuration is one in which the precursor is separated from the absorbing substrate by a thin transparent or non-energy absorbing film, for example a silicon dioxide film on the order of up to several thousand Angstroms. The energy absorbing film or substrate rapidly transfers the heat due to the absorbed energy to the precursor. The portion of the precursor not exposed to the energy (light)/heat source is rinsed off with a solvent such as ether, leaving only the patterned small molecule organic semiconductor. Depending on the desired feature size, the appropriate focal spot size of the energy and scanning range can be adjusted.
Other energy sources can also be used such as electron beams, miniature thermal heaters, etc. although lasers are a preferred energy source for precursor patterning. Previous art has described the use of a laser to amorphize and crystallize pentacene for use as an optical memory but not derived from a precursor nor used for maskless produced channel regions for semiconductor devices (U.S. Pat. No. 5,707,779).
Printing and stamping of semiconductors has also been disclosed for production of semiconductor devices such as FET's. U.S. patent application Ser. No. 2002/0022299 A1 discloses Means for preparing an organic layer from a water soluble photosensitive organic layer. In that invention the desired patterning of the semiconductor is in part the result of using an aqueous based developer to selectively remove unexposed regions for producing the desired patterning. These features and procedures of this cited application are not claimed as part of the present invention.
Accordingly, it is an object of the present invention to overcome the problems of the prior art by providing a method and system for producing a maskless patterned small molecule organic semiconductor layer on a substrate from a precursor of the small molecule organic semiconductor.
The present invention provides a method of producing a substrate with a patterned small molecule organic semiconductor layer from a precursor with dimensions as small as one micron in width. These large crystallite semiconductor layers have relatively large mobilities. The resulting semiconducting patterning capability has applications in photonic and microelectronic devices such as organic photodiodes and organic light emitting diodes.